Recent ultrasonic diagnostic devices have a function of displaying an ultrasonic tomographic image (B mode image) obtained from a reflected echo resulting from a difference in reflectivity between tissues to indicate a tissue structure in a living body, as well as a function of measuring a blood flow velocity and a blood flow rate in a blood vessel and a heart using ultrasonic Doppler blood flowmetry or measuring a velocity or amount of motion of tissue portions such as motion of a cardiac wall and displaying information about them.
Further, recently, ultrasonic waves have begun to be used to measure hardness of tissue portions for tissue diagnosis. This is because hardness, i.e., an elastic property in a rough area is deeply associated with its pathological state. For example, it is known that a disease area is harder than a normal tissue in the case of sclerosing cancer such as breast cancer or thyroid cancer, liver cirrhosis, arteriosclerosis, and the like. Conventionally, information regarding such hardness is obtained from palpation. However, it is difficult to express objective information using palpation and palpation requires experienced doctors. To address this, in recent years, the following method has been utilized to determine hardness of a tissue. That is, the hardness of a tissue is determined from the magnitude of a strain caused in the tissue and measured using ultrasonic wave. The strain is caused therein by applying, from a body surface, static pressure or vibration with ultrasonic wave having a relatively low frequency. In particular, by incorporating the strain measurement function into an ultrasonic diagnostic device and simultaneously visualizing a tissue structure obtained by a conventional B mode image and a strain distribution, the disadvantage of palpation can be overcome and the tissue structure and the distribution of hardness can be compared and contrasted. Accordingly, a larger amount of information for diagnosis can be provided. Ultrasonic diagnostic devices having such a function have begun to be used mainly for diagnosis for mammary gland area.
The above-described velocity measurement for blood flow or the like and strain measurement with the use of ultrasonic wave are both performed based on a function of measuring a displacement using ultrasonic wave. For example, in measuring a distribution in blood flow velocity, a distribution of displacements of a tissue portion at different time points is calculated from ultrasonic echo signals obtained at the time points. Then, a value of a displacement at each point in the distribution of displacements is divided by a time interval between the time points of the measurement. In this way, a velocity distribution is found. Similarly, a strain distribution is found as follows. That is, the value of the displacement at each point in the displacement distribution found using ultrasonic wave is differentiated with respect to a distance between the points.
As such, the above-described measurement for velocity in blood flow and various functions of tissue motion, as well as the strain measurement for inspecting the hardness of a tissue are based on the function of measuring a displacement using ultrasonic wave. Hence, in recent ultrasonic diagnostic devices, such displacement measurement using ultrasonic wave has become an important technical issue.
A conventional ultrasonic diagnostic device transmits an ultrasonic wave to a body tissue, and finds a displacement from an echo signal reflected by the tissue. For example, in a pulse Doppler method, a color flow method, and the like, a pulse train of an ultrasonic wave is transmitted and reflected echo signals corresponding to different transmission pulse ultrasonic waves of the pulse train are compared to detect a time deviation (phase difference) between the reflected echo signals at each portion. This time deviation is caused due to a difference between times taken for the reflected ultrasonic waves to reach an ultrasonic reception location. The difference between the times is caused depending on a difference between the locations of a tissue in the ultrasonic propagating direction. The tissue has generated reflection echoes for portions thereof on the received echo signals corresponding to the different transmitted pulses. Hence, by multiplying this phase difference (=time difference) by acoustic velocity in the tissue, a difference between the locations of a corresponding portion, i.e., displacement, in the ultrasonic propagating direction is detected for the received echo signals of the different transmitted pulses. In actual detection of such a phase difference, a process of calculating is likely to be performed to detect the phase difference using frequency analysis between the reflected echo signals corresponding to the different transmitted pulses, such as correlation arithmetic or FFT (Fast Fourier Transform).
Further, in a continuous-wave Doppler method, a velocity component in the ultrasonic propagating direction is directly calculated using frequency analysis or the like, based on such a fact that a continuous ultrasonic wave is transmitted and reflected echo signals received are changed in wavelength by the velocity component in the ultrasonic propagating direction of the reflecting tissue due to an ultrasonic Doppler effect. In this case, the velocity component can be thus measured directly. In order to find a displacement, the velocity component may be integrated by a corresponding time interval.
These methods find a displacement or a velocity in the ultrasonic propagating direction. However, an actual tissue displacement does not necessarily take place only in the ultrasonic propagating direction. Hence, generally, during diagnosis, based on a B mode image visualized simultaneously, the ultrasonic propagating direction is matched to a direction of displacement estimated from a tissue structure, or the estimated direction of displacement is corrected with respect to the measured displacement or velocity. However, generally, it is difficult to estimate the direction of displacement at each portion in advance, which results in an error caused by the estimated direction.
Proposed to overcome such a disadvantage is a method of measuring two-dimensional or three-dimensional displacement and velocity using ultrasonic wave.
For example, Fox has proposed a method of providing two velocity components by means of compound scanning performed from two different locations in “Multiple crossed-beam ultrasound Doppler velocimetry”, IEEE Trans. Sonics Ultrason., Vol. 25 pp. 281-286, 1978, (Non-Patent Document 1). However, this method suffers from a practical problem because it requires a transducer element array with a large aperture. An aperture of a transducer element array actually usable therefor has disadvantages such as limited visual field and insufficient precision provided by velocity composition.
Further, Trahey et al., has proposed a two-dimensional speckle tracking method based on a frame-to-frame correlation analysis, in “Angle independent ultrasonic detection of blood flow”, IEEE Trans. Biomed. Eng., Vol. 34, pp. 965-967, December, 1987, (Non-Patent Document 2). This method is to perform two-dimensional correlation arithmetic between frames of B mode images captured at different time points, or two-dimensional correlation arithmetic of obtained sequences of reflected echo signals. In many ultrasonic diagnostic devices currently available, most part of a process for obtained signals, in particular, beam forming, detection, and the like are performed by means of digital processing. Hence, in the case of performing such two-dimensional correlation arithmetic, it is preferable to perform digital two-dimensional correlation arithmetic in view of precision and reliability of the process, compatibility to existing systems, and the like. Generally, in the digital two-dimensional correlation arithmetic, processing speed thereof, circuit scale for the process, and the like are greatly dependent on the number of data sampling. Hence, a smaller number of data sampling is preferable. Conversely, in the case of using the two-dimensional speckle tracking method, the number of data sampling is determined by its displacement detection precision and a measurement area. Hence, in order to increase the precision in displacement detection in a specific measurement area, the number of data sampling should be increased. Normally, when precision in displacement detection in each of two-dimensional directions is multiplied by N, the number of data sampling is increased by N2. For example, in a normal ultrasonic diagnostic device, signals with ultrasonic reflection echoes are received at a relatively high sampling rate when creating B mode images. Hence, for the imaging thereof, the number of sampling (detection) is reduced. Further, an interval between scan lines of ultrasonic beams is relatively large. Hence, for the imaging, the number of scan lines for displaying is increased in the ultrasonic scan line direction by interpolation or the like. When obtaining two-dimensional correlation between frames of B mode images displayed in accordance with the two-dimensional speckle tracking method, precision for displacement is the same in degree as that for the image on pixels. In this case, the precision for displacement is reduced because the number of sampling is reduced in the direction of reflected ultrasonic echo reception signals measurable using a normal Doppler method or the like. Hence, with this method, a displacement component in a direction different from that in the conventional art can be detected, but the precision of detecting a displacement component in the direction in which it can be attained in the conventional art is decreased greatly. Further, the two-dimensional speckle tracking method may be performed using reflected ultrasonic echo signals prior to detection. In this case, the precision of detecting the displacement component can be secured in the direction in which it can be attained in the conventional art. However, the number of data sampling becomes enormous, with the result that the two-dimensional speckle tracking method cannot be implemented in the scale of the conventional ultrasonic diagnostic devices. Thus, the two-dimensional speckle tracking method is only applicable to detection of motion of a tissue portion displaced relatively greatly, disadvantageously.
As a relatively new method for two-dimensional displacement measurement, Japanese National Patent Publication No. 2001-503853 (Patent Document 1) discloses a method for providing lateral modulation in a received beam pattern by means of a coherent process using two sub-apertures provided in a transducer element array. In this method, spatial modulation is provided to reception sensitivity in two directions by means of interference of the two reception sub-apertures, thereby allowing for detection of displacement components in the two modulation directions. This method can provide an ultrasonic wave with a wavelength spatially modulated in its propagating direction substantially as much as in the direction of a reflected ultrasonic echo in which the detection of displacement can be attained in the conventional art. Hence, a displacement component can be measured in a direction orthogonal to the ultrasonic propagation without greatly decreasing the precision for displacement in the ultrasonic propagating direction as compared with the precision for displacement in the conventional art. However, in this method, spatial resolution is decreased, disadvantageously.
U.S. Pat. No. 6,859,076 (Patent Document 2) discloses a method for improving measurement precision in the method described in Japanese National Patent Publication No. 2001-503853 (Patent Document 1). However, in the method disclosed therein, a process including calculation of 4th order moment for reflected ultrasonic echo reception signals is substantially performed. Accordingly, an amount to be processed is increased. This makes it difficult to implement the method in a conventional ultrasonic diagnostic device, disadvantageously.
Further, a method different from that of U.S. Pat. No. 6,859,076 (Patent Document 2) in transmission/reception beam formation is described in Liebgott, et al., “Beamforming Scheme for 2D Displacement Estimation in Ultrasound Imaging” EURASIP Journal on Applied Signal Processing 2005: 8, pp 1212-1220, August, 2005 (Non-Patent Document 3).